Everyday the Sun pours down onto the Earth a vast quantity of radiant energy many many times greater than the total now used by Man. Some of this energy, together with carbon dioxide and water, Nature traps in trees and other plants by conversion into oils and fats. As they exist in the plants these materials are unsuitable for use as diesel engine fuels. However. the energy trapped within the oils and fats of the plants can be recovered, in part, by breaking down the complex chemical structures of the oils and fats into a mixture of the water-soluble trifunctional alcohol, glycerol, and various fatty carboxylic acids. Thereafter by conventional procedures the fatty acids can be esterified with methanol to yield biodiesel. The breakdown reaction of oils and fats is usually conventionally accomplished in batch reactors by protracted heating of the oils and fats with water to which a basic catalyst, such as sodium hydroxide or potassium hydroxide, has been added. The time of the breakdown reaction is several hours because batch reactors are intrinsically inefficient in comparison to continuous reactors. This batch reaction time can be somewhat reduced by replacement of the hydroxide catalyst with sodium methylate or potassium methylate. These catalysts are difficult to ship because of their ready reaction with moisture. Storage is also a problem for the same reason. Manufacture of the catalysts at the actual location of the oils and fats breakdown process by the reaction of elemental sodium or potassium metals with methanol is troublesome and hazardous. Moreover, the use of either of these catalysts leads to the formation of the sodium or potassium salts of the fatty acids generated by the breakdown of the oils and fats. Such salts are known as soaps and can cause costly chemical engineering difficulties such as foaming as well as emulsion formation. Finally, the water-soluble sodium or potassium salts of the fatty acids must be destroyed by treatment with mineral acids to liberate the free water-insoluble fatty acids for subsequent reaction with methanol to form the so-called biodiesel. This acid treatment step produces the sodium or potassium salts of the mineral acid as a waste product which must be disposed of subsequently. The problem facing chemical engineers has been how to achieve these several sequential chemical reactions on a large-scale, commercially practical, and energy efficient way.
Nowadays, everyone is aware of the desirability of having new domestic sources of liquid fuels for diesel engines and as a result inefficient conventional processes for the conversion of oils and fats to biodiesel have been developed.
One of the most intriguing and environmentally sound approaches to breaking down molecules is simply to use water alone, heated to its supercritical state. About a decade ago this chemical-free technology was comprehensively discussed in an English language review by P. E. Savage (Chem. Rev. 1999, 99, 609). Since then few modern reviews have appeared. However, numerous articles, mostly from Japan and China, have appeared each year dealing with the reactive power of supercritical water. All of these publications emphasize that when water is heated to 374.4 C or above, the pressure concomitantly generated is 217.7 atm and above. The water then becomes a powerful new reactive solvent. Temperatures above 400 C seem to make the water even more effective in its new role. For example, it now dissolves nonpolar substances such as oils and fats.
These and numerous other similar reactions (J. A. Onwudili & P. T. Williams, Chemosphere 2009, 74(6), 787) demonstrate clearly that chemical bonds can be broken down by treatment with supercritical water only, without the use of any catalysts. Apparently the water and substrates may undergo the water gas reaction and hydrogen is released to combine with the molecular fragments from the substrates. This has actually been demonstrated by the use of deuterium oxide in place of water and the consequent finding of deuterium in the fragments. However, since nearly all water-substrate reactions have been run in a batch mode on a very small scale, the chemistry so elegantly elucidated does not provide answers to the questions necessary for the future development of a commercially-sized, practical, continuous, supercritical water-based process.
As is commonly understand by those with backgrounds in chemical engineering, petroleum-based diesel fuels are produced from the fractional distillation of crude oil between 200 C (392° F.) and 350° C. (662° F.) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule. Diesel fuels are approximately similar to fuel oils used for heating (fuel oils no. 1, no. 2, and no. 4). All fuel oils consist of complex mixtures of aliphatic and aromatic hydrocarbons. The aliphatic alkanes (paraffins) and cycloalkanes (naphthenes) are hydrogen saturated and compose approximately 80-90% of the fuel oils. Aromatics (e.g., benzene) and olefins (e.g., styrene and indene) compose 10-20% and 1%, respectively, of the fuel oils. Fuel oil no. 1 (straight-run kerosene) is a light distillate which consists primarily of hydrocarbons in the C9-C16 range; fuel oil no. 2 is a heavier, usually blended, distillate with hydrocarbons in the C11-C20 range. Straight-run distillates may also be used to produce fuel oil no. 1 and diesel fuel oil no. 1. Diesel fuel no. 1 and no. 2 are similar in chemical composition to fuel oil no. 1 and fuel oil no. 2, respectively, with the exception of the additives. Diesel fuels predominantly contain a mixture of C10 through C19 hydrocarbons, which include approximately 64% aliphatic hydrocarbons, 1-2% olefinic hydrocarbons, and 35% aromatic hydrocarbons.
Accordingly, and although some progress has made with respect to the development of renewable systems for the manufacture of non-petroleum diesel fuel, there is still a need in the art for new and improved machines, systems, and methods for the continuous conversion of plant or animal material into liquid transportation fuels. The present invention fulfills these needs and provides for further related advantages.